In recent years, display devices including thin-film transistors and capacitors have been put to practical use. Examples of the display devices are a liquid crystal display device and an organic electroluminescence display device.
As an example of the structure of the thin-film transistor, there is known a top gate structure including a polysilicon semiconductor layer. In addition, a structure in which a capacitor is formed by using a polysilicon semiconductor layer is known. Besides, there is known a structure in which a capacitor is formed by using a Cs line which is in the same layer as a gate line, and a drain electrode which is opposed to the Cs line via a gate insulation film.
Recently, as a structure of a thin-film transistor, a structure including a layer of an oxide semiconductor typified by indium gallium zinc oxide (IGZO) has been vigorously studied. The oxide semiconductor, compared to polysilicon, enables uniform formation over a wide area at relatively low temperatures. In addition, in the fabrication step of forming an oxide semiconductor, it is not necessary to perform a step of crystallization by a laser anneal method which is required when a low-temperature polysilicon semiconductor is formed, or a step of decreasing the resistance of a diffusion region by an ion implantation method, or a step of activation anneal which is required when the resistance of a diffusion region is to be decreased. Thus, the step of forming the oxide semiconductor can lower the manufacturing cost, and does not require a high-temperature process. A bottom gate structure is dominant in thin-film transistors including oxide semiconductor layers.
Drawbacks in adopting the bottom-gate-type thin-film transistor are the difficult in enhancing the capability or fineness by decreasing the channel length, and the increase in gate-source parasitic capacitance of the thin-film transistor. This leads to a problem of degradation in display capability. In order to solve these problems, there has been a demand for practical use of a top-gate-type thin-film transistor including an oxide semiconductor layer.